The present invention relates to an anode in which an anode current collector is provided with an anode active material layer, and a battery using it.
In recent years, in connection with high-performance and multi-function of mobile devices, high capacities of secondary batteries, the power source for the mobile devices have been desired earnestly. As a secondary battery, which meets such a demand, there is a lithium secondary battery. However, in the case of using lithium cobaltate for a cathode and graphite for an anode, which is currently a typical mode for the lithium secondary batteries, the battery capacity is in a saturated state, and it is extremely difficult to greatly obtain a high capacity of the battery. Therefore, from old times, using metallic lithium (Li) for an anode has been considered. However, in order to put such an anode to practical use, it is necessary to improve efficiency of precipitation and dissolution of lithium and to control dendrite precipitation form.
Meanwhile, a high capacity anode using silicon (Si), tin (Sn) or the like has been actively considered recently. However, when charge and discharge is repeated, such high capacity anodes are pulverized and miniaturized due to significant expansion and shrinkage of the active material, current collecting characteristics are lowered, and decomposition reaction of the electrolytic solution is promoted due to the increased surface area, so that the cycle characteristics are extremely poor. Therefore, an anode in which the anode active material layer is formed on the anode current collector by vapor-phase deposition method, liquid-phase deposition method, firing method, or thermal spraying process has been considered (for example, refer to Japanese Unexamined Patent Application Publication Nos. H08-50992 and H11-135115, and Japanese Patent No. 2948205). According to such an anode, compared to a traditional coating type anode, in which a slurry containing a particulate active material, a binder and the like is coated, miniaturization can be inhibited, and the anode current collector and the anode active material layer can be integrated. Therefore, electronic conductivity in the anode becomes extremely excellent, and high performance in terms of capacity and cycle life is expected.
However, even in the anode in which the anode current collector and the anode active material layer are integrated, as the active material is expanded and shrunk, the anode current collector and the anode active material layer are separated, and it is difficult to obtain sufficient characteristics. Therefore, for example, a technique, in which by diffusing components of the anode current collector into the anode active material layer, contact characteristics between the anode current collector and the anode active material layer are improved and expansion and shrinkage in the diffusion region are inhibited has been reported (for example, refer to International Publication No. WO01/029912). Further, a technique in which impurity is added to an anode active material layer to obtain a gradient structure, in which the impurity concentration is changed in the thickness direction has been reported (for example, refer to International Publication No. WO01/031721).
However, even in the case of using such techniques, there have been disadvantages that it is difficult to sufficiently inhibit expansion and shrinkage of the anode active material layer, and to improve battery characteristics such as cycle characteristics.